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What is Dark Matter?


What if there was an invisible substance that interfered with all things visible? How could we possibly detect the invisible? For decades scientists have been trying to understand discrepancies in data which led to the idea of invisible matter interacting with ordinary matter, but gravitationally. This invisible matter is now known as dark matter, but now understanding what it is and its purpose is of great importance. The most recent and sound theory states that it is non-luminous, non-baryonic. Non-luminous describes its nature of not interacting with light, which makes it invisible. While non-baryonic describes the theory that dark matter is comprised particles other than baryons (baryons include protons and neutrons, neutrinos are an example of a non-baryonic particle). Dark matter is thus a new hypothetical form of matter and is often confused with anti-matter, which was discovered in 1932 and is made up of antiparticles.



When was it established?


The idea of objects being invisible to us was not a new concept. During the renaissance period (14th to the 17th century) technology was still very much primitive, much of it relied on manual labour and observing the night sky could only be done with the naked eye. This meant that astronomical bodies were mostly invisible, save for some planets of our solar system. Galileo Galilei used his telescope to observe the night sky and discovered other cosmic entities that were once previously "invisible". This indicated that there was much of the universe that had yet to be discovered and in retrospect, showed that our technology would have to develop significantly for us to be able to detect unseen matter. Another instance of this was seen years down the line in 1844 and 1846. The German astronomer, Friedrich Bessel, deduced the existence of unseen objects based on their gravitational effect on two stars, Sirius and Procyon, as their paths deviated slightly. Urbain Le Verrier met a similar problem as discrepancies were found between Mercury’s observed perihelion precession rate and the predicted precession rate calculated through classical mechanics. A perihelion is the point in an orbit an object is closed to the orbiting star, in our case, when the Earth is closest to the Sun in its orbit. The problem Le Verrier encountered was that the perihelion was advancing at a rate that did not match his prediction. To explain this, an unseen planet called "Vulcan" was proposed which disturbed Mercury's regular path. Furthermore, Le Verrier had also predicted the existence of Neptune in 1946 based on abnormalities in Uranus' path after it had completed a full orbit in 1847 (since its discovery in 1782).

Unseen objects like these were often referred to as dark matter. However, this form of dark matter consisted of two words; the adjective dark and the noun matter, which together meant astrophysical objects that were too faint to detect.

Lord Kelvin (1904) proposed that there is a relationship between the size of a galaxy and its velocity dispersion, but was also the first to make an estimate of the amount of dark bodies in the Milky Way. Note that velocity dispersion is a distribution of the velocities of objects within a galaxy, compared to the mean velocity. This hypothesis led to Ernst Öpik, an Estonian astronomer and astrophysicist, as well as Henri Poincaré to separately test Kelvin's belief. Their individual models on the motion of stars resulted in Poincaré stating that "the amount of dark matter present was likely less than or similar to that of visible matter", to with Öpik agreed. Several other renowned physicists corroborated with this belief through the use of calculations and probable values.


However, in 1933, when comparing the observed velocity dispersion with his own predicted value, Fritz Zwicky found a worrying discrepancy. The observed value was over 10 times greater than his predicted value (1000km/s and 80km/s respectively). The cause for such a difference was due to the presence of unseen mass which contributed to a greater density and thus a greater velocity dispersion. This discovery prompted him to doubt that the amount of dark bodies was far less than luminous matter, but in fact far greater, as other physicists began to find similar results. Many astronomers had their own theories as to why this occurred, but the most reasonable argument, at the time, was by Erik Holmberg who proposed that the "invisible" mass is due to temporary "galaxies that were not bound to the cluster, but merely under its gravitational influence".


Another proposition, which was deemed false, was brought to light by Victor Ambartsumian in the late 1950s. He rejected the idea of dark bodies and stated that clusters were unstable and are expanding. This Theory of Instability was discussed in a large meeting and later disproven due to the fact that if it were accurate, galaxies would have evaporated long ago. In the midst of all these propositions, another discovery further added to the mystery and incentivised physicists. Kent Ford and Vera Rubin were looking into galactic rotation curves, which compared the velocity of objects in a galaxy to the distance from the centre of the galaxy. Similar to Zwicky, a large discrepancy was found between the predicted curve and the observed curve. The former curve suggested that the greater the distance from the centre, the lower the velocity. However, the observed curve plateaued at the peak thus implying that at greater distances the velocity remained similar.


Although this experiment's data was not referenced at first and received little attention, similar results appeared later on which led to the conclusion that the overall idea of dark matter is the cause for the discrepancies with velocity dispersions and rotation curves. An alternative possibility was that the missing mass was ionised gas. However, this was disproven, but later led to the idea that dark matter did not consist of dark bodies after all. Following the examination and analysis of primordial light element abundances (the number of specific elements that made up stars from the early universe) Herbert Rood and other researchers found that dark matter actually favoured a non-baryonic nature.


This was a significant discovery and changed the way that dark matter was approached. It opened new doors and the possibility that new exotic unseen particles were present while also linking particle physics with astronomy and astrophysics. Particles that could be possible candidates were later proposed and included existing particles such as neutrinos and hypothetical particles such as WIMPS (Weak Interacting Massive Particles) and axions.

It was only in 1976 that neutrinos of a specific energy (10eV) were considered to be the missing matter. However, in the late 1980s numerical simulations were used to help understand how dark matter particles would evolve under the influence of gravity and help distinguish different “dark matter candidates”. Dark matter candidates were then separated into two classes: relativistic (hot) and non-relativistic (cold). Whether a particle was hot or cold depended on its speed, i.e. A third of the speed of light and above meant a hot particle and below was considered a cold particle (Speed of light = 3.00×108 m/s). The difference in the two classes of dark matter can be seen in the way and what they form around galaxies. In essence, hot particles collapse and lead to the formation of large dark matter halos but breaks down into smaller halos, through fragmentation. Cold particles, on the other hand, start off with small halos but merge to form large halos.


Soon after the use of numerical simulation existing neutrinos from the standard model and hot dark matter particles were ruled out as they were too light and could not account for most of the dark matter in the Universe.

Despite this, there is a possibility of other types of neutrino-like particles which could be cold.



Why is it important?


Dark matter also plays a vital role in uniting several areas in physics. Fields were once separated and knew nothing about other fields. Cosmology in particular, the study of the universe as a whole and its origin, was once considered a "fringe" science and was often undermined by others. However, because not much was known about dark matter, and as theories about its existence progressed, more and more fields became more active and involved. One particular group were the Particle physicists, after unknown sub-atomic particles were introduced as a possible constituent of dark matter. Lines became blurred and hybrid fields were created such as the particle-astrophysicists, which symbolised a more united community rather than several isolated communities. They were all brought together under one aim which could provide answers to several questions in the individual fields.



Detecting dark matter: direct detection


One method of detecting dark matter is called 'direct detection'. This method was based on the work of Andrzej Drukier and Leo Stodolsky in 1984 who proposed to detect neutrinos upon colliding with a nucleus. The proposed detectors were small, consisting of micron-scale super conducting grains, that could measure the kinetic energy of the nucleus after the collision.

In 1985, Mark Goodman and Ed Witten realised that similar detectors could be used to search for dark matter. The scale of the experiment was far too big for it to be able to detect most dark matter particles but it was enough to be able to test the WIMP particle hypothesis. This has allowed us to narrow down further, predicted models as they are proven to be incorrect.

Another possible candidate for dark matter is axions, which we have been searching for ever since they were proposed by  Roberto Peccei and Helen Quinn in 1977. The most notable experiment for detecting these particles is the ADMX (Axion Dark Matter eXperiment) which began in 1983 by Pierre Sikivie. This occurs in a metal cylinder that uses magnetic fields and which should be able to convert axions into photons in the microwave region of the electromagnetic spectrum. This experiment hasn’t yielded any evidence as of yet as to whether or not axions exist however, they have limited the range in which it could reside. If the axion is discovered, it would help us further understand dark matter and have a non-hypothetical candidate, which we could experiment with. It would also further our knowledge it other branches of physics such as the Big Bang Theory.



Detecting dark matter: indirect detection


Modern day our indirect detection comes from annihilation and decay. This consists of looking for possible products of the annihilation and decay of dark matter. This includes gamma radiation, neutrinos and cosmic rays. We look for these products in our atmosphere and beyond in the hope we can trace the source of the radiation, in the hope, the source is dark matter. This is a bit of an optimistic approach as there are many other possible causes for the gamma radiation from black holes or gamma decay amongst other possibilities.

Another way of identifying possible locations of dark matter in our galaxy and beyond is its gravitational effects. Effects such as gravitational lensing and the effects on the velocity dispersion in galaxies indicate an unseen mass, which accounts for significantly more than ordinary matter. Gravitational lensing is the bending of light around a mass, which makes far off galaxies, stars, planets, etc. seem a lot closer than they truly are.


Detecting dark matter: particle colliders


It is thought that dark matter can be produced in the Large Hadron Collider as dark matter is light enough however it would leave the collider undetected however we could infer their existence with the amount of momentum and energy it carries away with it. The UK helped build the LHC and continue to maintain it as only as far back as 2009, Viglen, a UK based IT company were responsible for the processing power for analysing the data collected from the LHC. There are also over 20 research groups from the UK working with the LHC. The LHC has not only advanced our technology but also advanced many fields of science from our electrical components



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